Heat exchangers including multiple flat tubes arranged parallel to each other, and fins joined to the flat tubes have been known. Patent Document 1 discloses a heat exchanger of this type (see FIG. 2 of Patent Document 1). This heat exchanger is a single-bank heat exchanger in which flat tubes are arranged in a single bank in an air flow direction. The heat exchanger includes an upper heat exchange region (principal heat exchange region), and a lower heat exchange region (auxiliary heat exchange region). The number of the flat tubes in the lower heat exchange region is smaller than that in the upper heat exchange region.
Further, Patent Document 2 discloses a double-bank heat exchanger in which heat transfer tubes are arranged in two banks in the air flow direction (see FIG. 3 of Patent Document 2). In this heat exchanger (evaporator), a refrigerant in the heat transfer tubes constituting the first bank and a refrigerant in the heat transfer tubes constituting the second bank flow in opposite directions. Thus, in such a heat exchanger, a superheated region 17 is formed in the heat transfer tubes of the first bank to extend from a right end of the tubes toward, but not to reach, a left end in FIG. 3. In addition, in such a heat exchanger, another superheated region 17 is formed in the heat transfer tubes of the second bank to extend from a left end of the tubes toward, but not to reach, a right end in FIG. 3.